For a site having high load, that is, a variety of electronic devices which consume power, a system of high capacity uninterrupted power supplies (UPSs) is typically employed to ensure that even in the event of an external power failure, uptime is ensured. Configurations for these systems may provide redundancy, so that in the event of a single UPS failure, full power may still be provided to the site. This is referred to as an N+1 system.
One such common configuration is a parallel redundant configuration. Several UPSs are connected in parallel to a single bus. Under normal conditions, all the UPS outputs work in parallel and share the load equally to supply the required power to the load. In the event of a failure of a single UPS, the remaining UPSs can supply the required load. In this configuration, the number of UPSs used is the total number needed to supply power to the site, plus one more. The UPSs are equipped with internal static bypass switches (SBS), which transfer the load to direct mains feed if the UPSs cannot supply the load (e.g., in the event of multiple faults, overload, etc.). The SBS inputs are fed from utility power transformers. According to this type of configuration, if the load exceeds the power which is supplied by a single utility power transformer, a complex arrangement of transformers and UPSs may be implemented in order to circumvent problems inherent with connecting mains power from several transformers to a single bus via the internal SBSs. More commonly, several groups of UPSs are each arranged in parallel. Each group is connected to a single transformer and a unique bus. According to this arrangement, in order to provide N+1 redundancy to the entire site, an additional UPS must be provided to each group of UPSs.